As disclosed, for example, in JP-A No. 2002-514550, electrically assisted bicycles are known which have a hub unit provided on the hub of a wheel of the bicycle and incorporating a motor or the like therein, such that the motor is driven when the pedals are subjected to a load not lower than a predetermined value, permitting the rider to run up a slope with ease.
The hub unit becomes heated when the motor is energized. When overheated, the motor can not be rotated with high torque, so that there is a need to provide a contrivance to release the heat from the motor.
With the construction disclosed in the above patent publication, however, the stator is hermetically enclosed in a motor housing and therefore is not allowed to radiate heat effectively.
Accordingly, it is practice to enclose a coolant in the hub and to cool the motor housing from outside to suppress the generation of heat from the winding. In the case where the coolant is enclosed in the hub, there is a need to provide a measure for preventing the leakage of the coolant to result in a cost increase. The provision of the coolant entails a corresponding increase in weight.
An object of the present invention is to provide a hub unit which is improved in motor cooling effect to prevent the motor from overheating, and a vehicle comprising the hub unit.